Paper product



Patented Aug. 5, 1941 PAPER PRODUCT Joseph H. Shipp, Wilmington, Del., assignor to I. du Pont do Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 1, 1938, l Serial No. 211,297

17 Claims.

This invention relates to paperproducts and, more particularly to grease-resistant paper and tomethods for its preparation.

For most uses to which paper i put some degree of grease-proofness is necessary. This applies to all grades and types of paper intended to take printing ink. For some purposes, such as the wrapping of greasy or oily food and for containers for oils, such as lubricating oil, more exacting requirements are to be met since in such cases the paper must be made completely greaseproof and the film should be tough enough so that it does not crack in handling thereby causing leaks. Materials such as gelatin, casein, starches, and vegetable proteins, which have been used for modifying the printing characteristics of paper, contribute a high degree of grease-proofness only when used amounts as to make the paper too stiff for many uses. This relatively thick coating, furthermore, cracks readily and is apt to become sticky in moist weather. Casein, because of its high adhesive value, water-resistance and polishing properties, has found extensive use in the paper industry, but this coating and sizing agent readily spoils through fermentation in aqueous systems and its use is particularly undesirable in 1 that solution must be effected by alkali which causes foaming, degradation of the paper through residual alkali, and alteration of the color of most inks. Materials such as wax or hydrogenated castor oil have been proposed for oil containers but are not satisfactory because the film is not sufliciently tough to withstand handling,

Although materials useful in finishing papers have occasionally found use in the textile industry, the coating and sizing materials heretofore used furnish no guide to the selection of coata. ing and sizing materials which can be successfully used with paper. Because of the low wet strength of paper compared to that of textiles, the methods of processing paper are necessarily restricted to aqueous materials that have unusually good working properties. For example, low temperatures of treating baths are highly desirable in order to prevent weakening of the paper during the tub sizing or surface coating process. Also, the coating materials must have good brushing out properties because they must produce surfaces as smooth as wall paints. Materials with false bodies are highly undesirable. Foaming mixtures are highly objectionable because bubbles on the surface spoil the paper surface. A Wide temperature range over which the solution viscosity is fairly constant is valuable becaus it permits low temperature baths and coating applications.

This invention has as an object a new andimproved grease-resistantpaper. A further object in suchis the ,production of grease-proof paper which avoids the disadvantages which pertain to the use of the coating materials heretofore used or proposed for this purpose. A still further object is the provision of grease-proofing films having suflicient toughness to withstand bending without cracking. Other objects will appear hereinafter.

The foregoing objects are attained in accordance with the invention by providing paper and similar fibrous materials with an impervious coating of polyvinyl alcohol with or withoutpigments, softeners, wetting agents or insolubilizlng agents, as described more particularly hereinafter.

Polyvinyl alcohol is a tough, water-white, resin-like material, which is usually obtained by the hydrolysis of polyvinyl esters, such as polyvinyl acetate. The polymers that are useful in this invention are soluble in hot water. In cold water they go into solution with more or less difficulty, but when once dissolved remain in solution even in cold water. Polyvinyl alcohol can be made in a number of modifications of various degrees of polymerization, all of which are, to some extent, soluble in water. The more highly polymerized forms are characterized by being less soluble than the slightly polymerized forms. Partly hydrolyzed esters, e. g., incompletely deacetylated polyvinyl acetates, are soluble in water to greater or lesser extent, dependent on the number of free hydroxyl groups and merized alcohol itself or the partly hydrolyzed esters thereof that contain a sufficient number of free hydroxyl groups to render the composition soluble in water.

I have discovered that paper and other nonwoven fibrous articles may b rendered greaseresistant in accordance with the invention by applying to the surface thereof a solution of polyvinyl alcohol of suitable viscosity and concentration and evaporating the solvent therefrom to produce a thin'continuous well anchored film of polyvinyl alcohol over the surface. The most favorable concentration of polyvinyl alcohol solutions to be used for coating or spraying varies within considerable limits, depending on the intrinsic viscosity of the polyvinyl alcohol used, which in turn depends largely upon the degree of polymerization. The temperature, likewise, may vary considerably because polyvinyl alcohol does not require elevated temperatures to keep the solution smooth and free flowing. In some instances it may be desirable to increase the temperature of thesolution during application in order to remove air bubbles orto increase penetration. The coating operation may be carried out by immersing the article in a solution of polyvinyl alcohol, or in the case of containers and receptacles, an amount of polyvinyl alcohol may be introduced into the interior of the container by spraying or otherwise. The container is then tipped in order to flow out the excess solution into the reserve coating bath. For speciflc applications, other methods may be employed, such as brushing.

For efficient operation and to be suitable for spreading or brushing, the viscosity of the polyvinyl alcohol-pigment composition should be below about 40 seconds for a 50 cc. drainage from a Number cup. A drainage time of about seconds is preferable. The intrinsic viscosity of the unpigmented polyvinyl alcohol is determined on a 4% solution in water at C. Polyvinyl alcohols having an intrinsic viscosity of from 10 to 100 centipoises may-be used in the practice of this invention, but viscosities of from about 25 to about 90 centipoises are preferred.

In order to avoid excessive penetration of a polyvinyl alcohol solution into the material being, coated and to provide for more economic utilization of the polyvinyl alcohol, it has been found advantageous in coating porous papers to pre-treat the surface to be coated with an agent which tends to coagulate or insolubilize polyvinyl alcohol. Examples of materials which are suitable for this purpose are various inorganic compounds such as borates, perborates, silicates, chromic acid, chromates, dlchromates, alum, and ferric chloride; and organic hardening agents such as tannins, formaldehyde and other aldehydes, azo dyes such as Congo red, and the like.

If desired, the normal flexibility of the polyvinyl alcohol films may be increased by incorporating in the polyvinyl alcohol solution, a suitable softening agent or plasticizer. Among the plasticizers which are suitable for this purpose may be mentioned: polyhydroxy compounds such as glycerol, polyglycerols, glycols, polygly- I methods for practicing the invention:

Example I A 20-pound unfinished glassine paper was treated on both sides by passing the sheet through a 2% solution of polyvinyl alcohol, containing, 0.5% invert sugar, and expressing excess solution between squeeze rolls. The moist sheet was stored in roll form until the moisture was uniformly distributed throughout the sheet and the paper was then polished by passage between calender rolls. The last roll was heated to dry the sheet. The solids absorption was 2.3% and turpentine resistance was 240+ minutes. The transparency and surface glosswas greatly improved over that of a sheet from the same lot of paper which had'been treated in a similar manner with a glycerine solution to give 7% glycerine absorption. Its tendency to curl in the printing press was considerably less than that of the (color depth and uniformity).

glycerine softened paper. The turpentine resistance of a glyccrine treated sheet was 210 minutes and of a water treated sheet 29 minutes.

Example II A 50-pound borid paper was tub-sized on both sides with a 2% polyvinyl alcohol solution, and water was removed by passing over drying cylinders. The solids absorption was 2.8%. In comparison with the same base paper which had been tub-sized with gelatin to a 3.5% solids absorption, the fold test, retention of color and printing properties were superior.

Example III A solution containing 20% solids was prepared by dissolving 120 grams of polyvinyl alcohol (intrinsic viscosity 40 centipoises) in 480 grams of water. This solution was diluted to a concentration of 10%, whereupon 0.6 gram of an alcohol sulfate was added to improve surface wetting rate. The interior surfaces of cardboard ice cream cartons were coated with this solution by pouring an excess of the solution into each container, then tipping and turning the container to flow the solution over the entire interior surface. The containers after being placed in an inverted position to permit the excess coating solution to drain oif, were placed in an oven at a temperature above C. until the coating became thoroughly dry. These-treated containers were filled with lubricating oil of medium viscosity, and did not show any evidence of oil penetration after a period of 11 days storage at 50 C.

Example IV The following composition was used to coat both sides of a while 40-pound clay filled and sized paper of book grade with an application of 30% solids on each side.

Clay (coating paper grade) 100.0

Polyvinyl alcohol solution 6% (intrinsic' viscosity 68 centipoises) 55.5 Water 188.5

. The clay was made into a slurry by stirring with water. The polyvinyl alcohol solution was added to this with stirring and water was then added to a solids content of 30.2%. The viscosity of the coating composition was 13.3 seconds at 25 C. for a Number 10 cup. The coating was applied by a coarse brush and smoothed with a fine brush. The sheet was dried 'by passing through a heated chamber. by passing through a stack made of alternate chilled iron and paper covered rolls. In comparison to the same paper base, coated with the same coating clay containing 15% casein (based on clay weight) the polyvinyl alcohol coated sheet was superior in respect to gloss, smoothness, crack resistance, brightness and ink reception The natural ink colors were not modified by this coating as was the case of the casein coating where blue colors were changed to a red cast due to alkalies in the casein.

Example V A 40-pound white book paper base, which had been coated on both sides with a pigmented casein composition to 65 pounds, was printed in a magazine printing press. After the ink was dried, the printed sheet was coated with a 2% solution of polyvinyl alcohol (viscosity '70 centipoises) by roll application and dried by passing over steam heated cylinders. Solids absorption It was calendered on each side was 1.8% based'on printed sheet weight. This sheet was then coated with a pyroxylin lacquer to a solids absorption of 5.0% on each side. The polyvinyl alcohol coating prevented ink smudging andproduced afinish comparable in smoothness to a sheet (not coated with polyvinyl alcohol) which was coated with a pyroxylin, lacquer to 12% lacquer solids on each side.

Example VI absorption and the same calender finishing.

The coating composition consisted of Parts Clay 100.0 Polyvinyl alcohol 4.0 Water 796.0

This invention is applicable to any type of paper wherein it is desired to develop either a high or a low degree of grease-proofness. The invention is of particular advantage for treating paper designed to take printing.

The term paper" is used herein in its generic scope and includes, for example, such types as newsprint, hanging, catalogue, poster, book (coated and uncoated) cover, writing, wrapping, and board paper and paper that has been made in molds to certain shapes or that has been converted into containers after sheet formation.

. Other paper like materials such as that prepared from asbestos and fiber glass may be treated with polyvinyl alcohol to impart grease-resistance.

An aqueous solution of the polyvinyl alcohol may be applied to paper with or without pigment to produce various degrees of imperviousness to grease, oils, and inks. The unpigmented coating compositions may be applied to the paper surface by spreading with a knife, followed by drying. It is finished to a smooth polished surface by calendering at room temperature or at elevated temperatures. Sprays, coating rolls or tub size rolls may be used to produce desired results depending on the condition of the paper base and the degree of impervi ousness desired. The coating compositions may be applied to the sheet at various stages after sheet formation, such as the wet presses, in the tub size press after partial drying in the water boxes of the calender rolls, or by. a special coating process of the dried finished paper base.

Pigmented coatings of these adhesive compositions may be applied 'in large amounts to paper by spreading, then smoothing with soft brushes, drying and calendering. For high grade coated papers that require good printing properties and smoothness about 4 parts of the adhesive is used per 100 parts of clay. However, some pigments are satisfactory when used with a lower or higher ratio of adhesive. Comparable coatings in which starch is used alone as adhesive require from 22 to 30 parts of a very low viscositystarch per 100 parts of pigment.

Clays, chalks, calcium sulfite, titanium dioxide, titanium dioxide extended with low hiding power pigments, lithopone and colored pigments may be used satisfactorily. Highly alkaline pigments such as satin white cause foaming of the coating composition and require a higher ratio of adhesive. Pigmented coatings may be applied to each side of the paper in amounts up to half the weight of the paper base for each side without deteloping blisters or flaking off. Amounts as low as 1 pound per 3,000 square feet of the pigmented coating applied to one side of the paper will produce a uniform surface with appreciable resistance to oil base ink. Foaming is reduced by keeping the coating heated to about 65 C.

The turpentine resistance which has been referred to in the examples was determined on the paper at 25 C., 50% relative humidity, by the following method: Sixteen samples 2" square were cut from representative areas of each roll of paper. These were placed on a smooth white paper which served as the telltale. One inch squares of blotting paper (kerosene absorption 350%) 0.050" thick were placed on each test sample. Then about 0.4 cc. of anhydrous turpentine colored with a red dye was applied from a burette to the blotting paper square and a 1" cube of brass was placed promptly on this. The test samples with blotter and brass cubes in place were moved along the surface of the telltale paper at intervals to determine time at which the tell-tale sheet was marked by the colored turpentine. The turpentine resistance was expressed as the average of all sixteen tests.

The present invention is advantageously applied to the treatment or grease-proofing of all the types of papers to which the previously used coating and sizing materials have been applied. By means of the present process substantially complete grease-proofness, as is evidenced by the unusually high turpentine resistance given in the examples, is obtained without loss of flexibility and without the surface film becoming sticky in warm weather. Among the unusually large number of valuable characteristics which I have found polyvinyl alcohol to possess and which make this coating solution particularly valuable for finishing papers and paper products, there may be mentioned the following properties of the coating solution and of the film formed thereby. The very slight change in viscosity which takes place between 10 C. and 75 C. permits easy application of the solution to paper. The fact that these solutions may be used either neutral or acid tends to reduce foam develop- -ment in coating and tub sizing operations. The

thermoplasticity of the air dried film facilitates surface polishing through calendering. The

toughness of the polyvinyl alcohol film permits more pressure in calendering than may be used with aqueous colloids that produce more brittle films. The high adhesive value of the film, which is four times that of casein, produces pigmented coatings that are brighter and more ink receptive than when starch, casein or glues are used as adhesives. The insolubility of the film in cold water produces coatings that are more serviceable than starch adhesive compositions. The alkali resistance of these films is particularly advantageous where wrappers are used for containers used to package soaps, etc.

The advantages of this invention are well illustrated by the oil and grease-proof paper containers which are obtained by the practice described herein. Paper, paper board, and similar materials have long been used for packaging a wide variety of solid commodities in as much as containers of this type are inexpensive, light in weight, are easily opened, and may be discarded and easily disposed of after use. However, paper containers have not hitherto been successfully utilized for the, packaging of liquid oils and greases or materials which exude oils of a permeating nature. Commodities such as butter. and cheese can be packed satisfactorily in the paper containers known heretofore only when these commodities are primarily wrapped in grease-resisting thin papers or regenerated cellulose, etc. because the oils and greases are apt to exude at some of the higher temperatures encountered in shipment. In view of this fact completely greaseproof papers that are capable of withstanding bending without cracking the film have-not been available for the packaging of such commodities. But by means of the present invention paper containers are readily produced which will withstand quite rough handling without developing leaks and in which oils and foodstuffs such as peanut butter can be packaged without first wrapping the contents in regenerated cellulose or other special papers and without absorption of the oil or grease from the contents into the container.

As many apparently widely different embodiments of this invention may be made without departing from be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. In the manufacture of grease-proof paper the step of applying to the surface of a paper having a porosity not exceeding book paper, a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from to 100 centipoises.

2. A process for the manufacture of greaseproof paper which comprises passing a paper having a I porosity not exceeding book paper through a dilute solution of polyvinyl alcohol. having an intrinsic viscosity, based on a 4% solution in Water at C., of from 10 to 100 centipoises,'removing the excess solution from the surface of the'paper, storing the paper to distribute the coating uniformly, polishing the surface of the paper by passing 'same through calendering rolls, and then drying said paper.

3. A process for the treatment of printed paper which comprises coating the printed sheet of paper having a porosity not exceeding book paper with a dilute solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25? C., of from 10 to 100 centipoises. then drying said coated surface and further coating the surface with a pyroxylin lacquer.

4. A process which comprises passing a pigment filled paper having a porosity not exceeding book paper through a dilute solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises, and friction calendering the surface of said paper so as to smooth same.

5. The process in' accordance with claim 1 characterized in that the polyvinyl alcohol has an intrinsic viscosity of from 25 to 90 centipoises.

6. The process in accordance with claim 2 characterized in that the polyvinyl alcohol has an intrinsic viscosity of from 25 to 90 centipoises.

the spirit and scope thereof, it is to 'I. A paper having a porosity not exceeding book paper and having had applied to at least one of its surfaces 9. solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises.

8. A substantially grease-proof product comprising paper having a porosity not exceeding book paper and coated with a surface film of polyvinyl alcohol, said polyvinyl alcohol being applied to the surface by passing the paper through a dilute solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises.

9. A paper having a porosity not exceeding book paper and having had applied to at least one of its surfaces a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 25 to centipoises.

10. A substantially grease-proof product comprising paper having a porosity not exceeding book paper and coated with a surface film of polyvinyl alcohol, said polyvinyl alcohol being applied to the surface by passing the paper through a dilute solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 25 to 90 centipoises.

11. In the manufacture of coated paper the process of applying to a surface of a paper having a porosity not exceeding book paper a composition comprising a solution of polyvinyl alcohol and pigments, said coating being capable of calender polishing at a temperature below 60 C., said polyvinyl alcohol having an intrinsic viscosity based on a 4% solution in Water at 25 C., of from 10 to centopoises.

12. A paper having had applied to at least one of its surfaces a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises, said surface having been pretreated with a coagulation agent.

13. In the manufacture of grease-proof paper the process which comprises treating a paper with a coagulation agent and applying to a surface of said treated paper a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises.

14. A book paper having had applied to at least one of its surfaces a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in Water at 25 C., of from 10 to 100 centipoises.

15. In the manufacture of book paper the steps of applying to at least one of its surfaces a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises.

16. A paper having the porosity of book paper and having applied to at least one of its surfaces a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 (3., of from 10 to 100 centipoises.

17. In the manufacture of grease-proof paper the step of applying to the surface of a paper having a porosity of book paper a solution of polyvinyl alcohol having an intrinsic viscosity, based on a 4% solution in water at 25 C., of from 10 to 100 centipoises.

JOSEPH H. SHIPP. 

